1. Field of the Invention
The present invention relates to a power supply apparatus, and more particularly to a power supply apparatus having a function to detect abnormality of a current flowing through a drive circuit incorporated in the power supply apparatus.
2. Description of the Background Art
Hybrid vehicles and electric vehicles have been attracting attention as they help the environment. The hybrid vehicle includes, as its power source, a direct current (DC) power supply, an inverter and a motor driven by the inverter, in addition to a conventional engine. Specifically, the engine is driven to generate power while a DC voltage from the DC power supply is converted into an alternating current (AC) voltage by the inverter to rotate the motor by the AC voltage and accordingly generate power.
The power source of the electric vehicle is a DC power supply, an inverter and a motor driven by the inverter.
A DC power supply being incorporated in the hybrid or electric vehicle is usually of high voltage so as to obtain high output. When a DC power supply of such a high voltage is used, however, the electric motor may seize up or burn out due to overheat at the time of overload. There may also be a risk of receiving an electric shock at the time of electric leakage. Thus, a safety device for preventing such risks is demanded (see, e.g., Japanese Patent Laying-Open Nos. 07-123504 and 2004-215316).
FIG. 13 is a block diagram showing a configuration of a safety device for an electric vehicle described in Japanese Patent Laying-Open No. 07-123504.
Referring to FIG. 13, the electric vehicle safety device 200 includes a switch 150 provided at a feeder line L from a DC power supply 110 to a load circuit 130. Switch 150 is configured to open/close in accordance with an external signal supplied from a protective circuit 140 to a drive circuit 151.
More specifically, in protective circuit 140, a current detector 141 detects a current flowing through feeder line L. An output of current detector 141 is amplified by a current detection circuit 142 and input to a control circuit 143. When a current value detected after a lapse of a prescribed operation time from the time point when the detected current value exceeded a rated current of load circuit 130 is still greater than the rated current, control circuit 143 drives an output relay circuit 144 so as to turn off a contact point r of switch 150 via drive circuit 151.
Clocking of the operation time is started at the time point when the current value detected by current detector 141 exceeded the rated current, and the operation time is set such that switch 150 is shut off when the current does not become equal to or lower than the rated current even after a lapse of the operation time. The operation time is set in accordance with the magnitude of the passing current, for example to be shorter inversely proportional to the increase of the current value. If the detected current becomes equal to or lower than the rated current within the operation time, switch 150 is not shut off, and clocking of the operation time will be started again when the detected current exceeds the rated current next time.
In the safety device for an electric vehicle shown in FIG. 13, following detection of a passing current exceeding the rated current, if the current does not become smaller than the rated current after a prescribed operation time from the detection, feeding to load circuit 130 is shut off, ensuring protection against overcurrent.
Further, feeding to load circuit 130 continues until the operation time elapses. That is, feeding to load circuit 130 is not immediately shut off even if load circuit 130 becomes temporarily overload. This avoids the undesirable situation where protective circuit 140 is activated to shut off the feeding when there is no abnormality.
In the abnormality determination method in FIG. 13, however, presence/absence of abnormality in the passing current is determined based on the rated current and the operation time that is decided uniquely in accordance with the magnitude of the passing current. This poses a problem in accuracy of abnormality detection in the following points.
Specifically, in the case where load circuit 130 of FIG. 13 includes an inverter and an AC motor, the passing current shows a current waveform of sine wave in a normal operation, whereas it shows a current waveform quite different from the original sine wave when there is abnormality in control of the inverter.
For example, the passing current at the time of abnormality may have a current waveform pattern that temporarily becomes considerably greater than the rated current, or a current waveform pattern that continuously flows in the vicinity of the upper limit of sine wave, going above and below the limit. When a large current exceeding the rated current flows through the inverter, the inverter will suffer temporarily a great load in accordance with the magnitude of the passing current and the time period during which it flows, possibly leading to breaking of the inverter. Meanwhile, when the current continuously flows in the vicinity of the upper limit of sine wave, the inverter will continuously suffer the load of the maximum level in the normal operation, which may also break the inverter. As such, in order to prevent breaking of the inverter, the current waveform that cannot be observed in a normal operation should be reliably determined to be abnormal.
Thus, in the above-described abnormality detection method, the abnormal current temporarily exceeding the rated current considerably is determined to be abnormal if the time period during which it flows exceeds a prescribed operation time.
Meanwhile, in order to detect the abnormal current continuously flowing in the vicinity of the upper limit of sine wave, the threshold value as the criterion for determination of abnormality needs to be set as low as the upper limit of sine wave, instead of the rated current. If the threshold value is set to the upper limit of sine wave, however, it is difficult to detect the abnormal current flowing in the vicinity of the upper limit of sine wave with accuracy, since when the passing current becomes lower than the threshold value within the time limit of the operation time, the clocking operation is reset and restarted when the passing current exceeds the threshold value again. That is, if the operation time is set relatively short, the passing current that temporarily exceeds the upper limit of sine wave, which is only a small load to the inverter, may be determined abnormal, whereas the passing current continuously flowing in the vicinity of the upper limit of sine wave may not be determined abnormal because of the reset of the clocking operation. As such, according to the abnormality determination method described above, the pattern of the abnormal current undesirable for the inverter and the result of determination of abnormality are not always consistent with each other.